Flexible Distributed Power Generation using the Industrial Trent Gas Turbine

Michael Welch, Siemens Industrial Turbomachinery Ltd., Lincoln, United Kingdom

Abstract

As the penetration of power generation onto the electricity networks from intermittent renewable sourcessuch as solar and wind increases, power generators and network operators are being forced toreconsider the design of power plant. Instead of large, centralised power plant operating at base load,today’s market requires flexibility of operation and fast responses to support these intermittent generationsources. In addition, as we move into a more carbon-constrained World, there is increasing pressure toswitch from coal and liquid fuels to natural gas as the primary fuel for power generation, bringingincreasing opportunities for gas turbines.

Traditionally gas turbine power plants have focussed on combined cycle (CCGT) configurations tomaximise full-load efficiencies using large ‘Heavy Duty’ gas turbines, typically in a 1+1 or 2+1configuration. However, these configurations are relatively slow to react to changes in grid powerdemand, have reduced efficiencies at part-load and increased operational costs due to the load cyclingand frequent starts. The large size of the turbines also leads to long plant construction times andmaintenance downtimes.

This paper examines the use of flexible power plant, both open cycle and combined cycle, based aroundthe Industrial Trent gas turbine for base load, intermittent and peaking applications. As an aero-derivativegas turbine, the Industrial Trent is ideal for power plant where frequent starts and stops are required, andoffers higher availability than a traditional combined cycle plant due to the core swap capability. Thepaper looks at various different combined cycle plant concepts to maximise operational flexibility to meetthe demands of Grid Operators, including an Organic Rankine Cycle option for a water-free solution, andexamines the economic and environmental benefits for some operational modes compared to aconventional combined cycle plant or flexible power solutions based on alternate technologies.

1.0 Introduction

An unbalanced power generation portfolio with the introduction of subsidised intermittent renewablepower generation with despatch priority is destabilising electricity markets and Grid systems by removingcapacity from transmission grids for firm generators. This is forcing fossil fuel power plants previouslydesigned for base load into cycling mid-merit and peak load applications. These intermittent operatingperiods are destroying the market pricing structures needed for long term investment decisions andincreasing the market exposure of generators through forward contract trading imbalance. These factorsare pushing large CCGT power plants out of the market, requiring grid operators to consider capacitypayment mechanisms that support inflexible assets and ensure security of supply, or develop strategiesto encourage operators of CCGT plants to operate assets as spinning reserve, negating the low carbonbenefits of natural gas. On top of this, environmental legislation and political uncertainties are removingexisting coal capacity and casting doubts over the future of nuclear power.

The historical generation portfolio of power plants has no correlation upon future generation requirementsdue to the increasing levels of intermittent renewable power generation. With electricity traded forward inhalf hour blocks, this favours intermittent renewable power generation, such as wind, as it enables moreaccurate forecasting of available generation than previous market designs where electricity was tradedhours in advance. However, intermittent renewable power generation requires back-up power generation

to balance the system and ensure supply security and system stability. Closing the trading gate to 30minutes in advance caused the existing power plants to provide this balance, leading to them becomingincreasingly stressed both economically and mechanically due to the cycling and faster starting timesrequired. Moves in some parts of the World to reduce the gate closure time still further to 10 to 20minutes will exacerbate the problem still further.

Figure 1: The impact of Renewable Power Generation on Grids

With natural gas proposed as the bridging fuel to a 100% renewable or zero carbon future powergeneration, the onus to provide intermittent renewables back-up capacity will fall on gas turbines and gasengines. Of the available technologies, aero-derivative gas turbines, like the Industrial Trent, are ideallysuited in meeting the challenges faced by the Grid Operators both today and in the future.

Modern industrial and heavy duty gas turbines were designed to provide high efficiency base loadcombined cycle operation with high exhaust and steam cycle temperatures. With thicker, heaviersections, load cycling and fast starts increases component thermal stress, leading to reduced componentlife and more frequent maintenance interventions. An increase in the operation and maintenance (O&M)costs and hence the cost of electricity generated is incurred. Gas engines, while able to start and stopfrequently and cycle without maintenance penalties, require relatively frequent maintenance interventionsand have high combustion emissions compared to aero-derivative gas turbines Gas engines also incur arelatively high parasitic loss to keep them warm and in hot standby mode in order to enable them to startquickly – a cold engine can take 10 to 12 hours to reach an initiate to start condition. In addition, due tothe maximum power output available of around 20MW per unit, larger power plant require large numbersof units, which while offering a great deal of flexibility, creates issues with synchronising to the grid andcan actually lead to an increase in the time required to bring a power plant to full load compared to asimilarly sized plants using a smaller number of gas turbines.

Figure 2: Firm Load Generation Profile

Aero-derivative gas turbines such as the Industrial Trent are by nature designed to achieve a lot of fastcycles, with no Equivalent Operating Hours (EOH) or stress factors applied for cycling operations, whilestill maintaining high open cycle efficiencies, which can be improved if required by utilising combinedcycle configurations. The Industrial Trent is capable of fast starts, high load ramp rates, full load rejectionand fast shutdowns, with no lockout period, while the core engine swap procedure ensures highavailability, making the Industrial Trent a highly suitable product that meets and exceeds the challengesfacing power generators and grid operators today.

2.0 The Industrial Trent Gas Turbine

The Industrial Trent gas turbine is the highest efficiency simple cycle gas turbine available today, withover 100 units sold for power generation and mechanical drive applications worldwide and a provenhistory from aircraft engine lineage.

The Industrial Trent gas turbine core is a three-spool design derived from the Rolls-Royce RB211 andTrent aircraft engines, with a performance lineage founded in Boeing 747, 757, 767, 777 (Trent 800) andTupolev TU204 applications. The Industrial Trent shares the same core as the Trent 800 aero engineproduct line initially developed during the 1990s for wide bodied aircraft, and now with more than 500engines in service and over 20 million service hours achieved.

The Industrial Trent is currently situated in the 42 to 66 MW market segment for industrial powergeneration (IPG) and a similar shaft power output in a mechanical drive (O&G MD) application. The initialproduct development industrialized three core areas: the lift fan was replaced with an aerodynamically

matched 2 stage axial Low Pressure Compressor configuration that delivered the same pressure rise asthe standard aero core; the Low Pressure Turbine was adapted, converting thrust output to rotationalenergy, by increasing the length of the last 2 blade rows of the 5 stage power turbine, and the combustionsystem was adapted to provide dry low emissions (DLE) and wet low emissions (WLE) alternatives for thecontrol of exhaust emissions.

Figure 3: Industrial Trent Gas Turbine core

The modular package design is optimized for Operation and Maintenance and minimal installation time.With the core exchange principle allowing offsite maintenance of the core turbine, overhaul outages arereduced to less than 2 days downtime.

Figure 4: Industrial Trent Generator Set Package

The Industrial Trent can be supplied in various configurations to meet the needs of a specific project. Foremissions control, both Dry Low Emissions (DLE) and Wet Low Emissions (WLE) are available on gasfuel, while only WLE is available for emission control on liquid fuel. WLE has the advantage of providing apower enhancement compared to a ‘dry’ turbine. For additional power enhancement, Inlet SprayIntercooling (ISI) can be supplied. ISI introduces water into the gas turbine air inlet to reduce the ambientinlet temperature and decrease the energy required for compression, resulting in an increase of bothpower and efficiency for ambient air temperatures above 7°C

A summary of the available configurations and gas turbine performance is shown in Table 1 below.

Configuration Power Output (MW) Efficiency (%)DLE 53.1 42.4

DLE + ISI 63.5 43.2WLE 66 4.4

WLE + ISI 66 41.5

Table 1: ISO performance data for 50Hz Industrial Trent configurations

3.0 Open Cycle Configurations

The Industrial Trent aero-derivative heritage and the modular package design make it a highly suitableunit for open cycle applications. The two main open cycle applications are peaking and cogeneration, butit is also possible with some modifications to utilise the Industrial Trent for synchronous condensingapplications.

3.1 Peaking and Cycling for Grid Support

In peaking applications or applications where frequent start/stop load cycling is expected, conventionaleconomic models are not applicable. The key evaluation criteria are not $/kW or heat rate, but the InternalRate of Return (IRR) on through life costs. Hence while an open cycle aero-derivative gas turbine is lessefficient than an optimised combined cycle plant utilising heavy duty gas turbines, the lower CAPEX andreduced O&M compared to a combined cycle plant costs outweigh the higher fuel consumption becauseof the limited number of operating hours.

Figure 5: Typical Power Generation Economics Chart

In these applications, fast response and high availability are two key attributes required by the generators.With the ability to start from cold and have 100% load available for despatch in less than 10 minutes, theIndustrial Trent offers probably the fastest power response time on the market. In addition, the IndustrialTrent is highly suitable for cyclic applications with no Equivalent Operating Hours (EOH) penaltyassociated with cycling or multiple daily starts for high stress operating cycle duty as shown below inFigure 6, and there are no lock-out periods after shutdown. The Industrial Trent has a 25000 houroverhaul regime and is capable of 7500 cold starts between overhauls. Additionally, the Industrial Trenthas very low black start and standby power requirements: the gas turbine will start up and commencepower generation on gas pressures of 22 barg with a power requirement of less than 350kW.

Figure 6: High stress operating cycle chart for an Industrial Trent peaking gas turbine unit

The chart below shows a typical start curve with a load ramp of 21MW/minute, but under certaincircumstances the ramp rate can be increased – up to 75MW/minute has been demonstrated on one siteenabling full load to be achieved in around 8 minutes from cold. This compares very favourably withreciprocating engines where OEM marketing material suggests 5 to 10 minutes to full load can beachieved on hot or warm starts, although it has to be noted that this applies to gas engines with poweroutputs of 20% to 40% of the output of the Industrial Trent.

Figure 7: Industrial Trent Start-Up Cycle

Several papers have been written on the so-called ‘Pulse Operation’, where the power plant is required tostart up, operate for just a few hours and then shut down again. Most economic comparisons for this typeof operation have been done by comparing gas engines, either in open cycle or combined cycle, with aconventional 1+1 or 2+1 CCGT utilizing heavy duty gas turbines. The long start up time and highmaintenance penalties for multiple starts (or the start costs) of the heavy duty gas turbines used in thiscomparison indicate the economics of pulse load operation favour the gas engine. However, with faststart up and shutdown times, high ramp rates and no start-up costs, the economic argument for utilisinggas engines rather than an aero-derivative gas turbine such as the Industrial Trent becomes much lesscompelling.

ICE CC ICE Trent DLEOC

Trent DLECCGT

Full Load NetEfficiency

% 44 49.2 41.87 53

Start-up time Minutes 5 50 10 40Shut-down

timeMinutes 1 20 5 20

O&M costs(2000 hours

per yearoperation)

EUR/MWh 5 5 3.50 4

Start-up costs EUR/MW - - - -

Table 2: Assumptions for Pulse Load Calculations for 100MW case

When calculating the cost and efficiency of a ‘pulses’ of different length (see reference 2), fuel andoperating costs for the start-up and shut down periods, which lie outside the settlement period (or pulse)were included in the calculation. Thus the faster the unit starts up and shuts down, the lower the fuel cost

and the greater the pulse efficiency. While the open cycle gas engine solution is slightly more efficientand potentially starts slightly faster than the Industrial Trent, the additional fuel used during theoperational pulses is compensated for by the lower maintenance cost of the gas turbine option. With aless obvious economic argument between the technologies, other factors such as emissions profile,availability, reliability and start reliability need to be considered.

The Industrial Trent economic argument in such applications can be improved by including combinedcycle configurations. These are discussed more fully in Section 4, but it is possible to achieve full plantload in a conventional steam combined cycle within 40 minutes from start-up, compared to the 50 minutesquoted for gas engines in combined cycle, and in 10 minutes using Organic Rankine Cycle technologykept in a hot standby condition. For a comparison of ‘steam’ combined cycle configurations, the IndustrialTrent has a faster start-up, lower maintenance costs and a higher efficiency solution that improves theoverall economics.

Figure 8: Approximate cost comparisons for different length pulses for gas engine and Industrial Trentconfigurations

From Figure 8, it can be concluded that for short ‘pulse’ operating periods an open cycle gas turbineconfiguration, and for longer ‘pulses’ a combined cycle gas turbine configuration, is the most attractiveeconomic solution.

3.2 Cogeneration

Cogeneration – the simultaneous production of power and heat from a single source – is often the mostenergy efficient way to produce the electricity and process heat required by industries or buildings. Byutilising a Distributed Generation principle and locating power generation closer to the actual consumers,the waste heat from power generation can be usefully recovered to provide this process heat while thepower generated displaces imported electricity from the grid. The high energy efficiencies achievable,often in excess of 80%, lead to a reduction in global greenhouse gas emissions compared to generatingthe power and heat separately.

0

5000

10000

15000

20000

25000

30000

35000

40000

4 hour pulse 8 hour pulse 14 hour pulse

ICE

Trent

CC ICE

CC Trent

Figure 9: Industrial Trent CHP plant at a cardboard manufacturing facility providing 50MW power and 200psig process steam

Cogeneration schemes are generally heat matched, and the electricity that is produced is a by-product.The site can either import electricity to make up any shortfall or export surplus power to generateadditional revenue. In peak power times, an economic decision may be to shut down the primaryproduction process and export as much electricity as possible.

Like all gas turbines, the Industrial Trent has the majority of its wasted energy in the exhaust gas stream.The relatively high exhaust gas temperature allows an efficient production of process steam. As theoxygen content in this gas stream is still relatively high, it is possible to install a duct burner between thegas turbine and the boiler to boost the exhaust gas temperature and increase steam production, asshown in Figure 10 below. This configuration increases overall energy efficiency compared to an unfiredsolution, and also allows the Cogeneration plant to have a degree of flexibility to optimise the matchbetween electricity production and heat production as process or market conditions change.

Figure 10: Industrial Trent WLE CHP/Cogen Capability Envelope

3.3 Synchronous Condenser

As well as providing peaking power, the Industrial Trent can also provide reactive power compensationduties. Simply by adding a clutch mechanism between the generator and the Low Pressure (LP) turbine,the gas turbine can be isolated from the rotating alternator and the alternator used to provide the reactivepower needed by the system operator. By carefully selecting the most appropriate locations in thenetwork, the flexibility of the Industrial Trent package to provide either kiloWatts or VARs at a singlelocation enables a system operator to maximise the use of this asset. This asset will optimise andstabilise grid system operation due to a large quantity of intermittent renewable power generation.

4.0 Combined Cycle Configurations

At times project economics are optimized by improving efficiency, especially for mid-merit power plantwhich operators require to run for longer operating periods than a peaking power plant. Combined cycle -utilising the waste heat from gas turbines, or gas engines, to produce steam to generate additionalelectricity using a steam turbine – is the well-proven choice to achieve this.

While the larger industrial gas turbines and the heavy duty gas turbines have been optimised forcombined cycle operation with high exhaust flows and temperatures, the Industrial Trent was designed foroptimum simple cycle efficiency and so has a relatively low exhaust gas temperature of around 450°C atfull load, compared to over 600°C for a heavy duty gas turbine. However, by utilising Once ThroughSteam Generator (OTSG) designs, the Industrial Trent can provide an efficient, flexible, economiccombined cycle power plant with fast start-up capability. The Industrial Trent is also one of the few gasturbines that can show an economic benefit from introducing a duct burner into the combined cycle plantdesign.

ISI can be employed as well throughout the whole operating period of the CCGT, or just during the initialstart-up phase to boost power until the steam turbine starts generating power.

4.1 Multi-Unit CCGT

The low exhaust temperature of the Industrial Trent requires the use of a low pressure, low temperatureHeat Recovery Steam Generator (HRSG). This has a number of benefits on the HRSG design, inparticular enabling the use of lower cost tube materials, thinner wall sections, reduced thermal stresswhen cycling the steam plant and eliminating creep. High cycle fatigue failure is a major concern in hightemperature steam plant, but in this design the highest steam temperatures are below the creeptemperature limits. This prevents the steam side of the CCGT from reducing the plant flexibility offered bythe gas turbine.

Combining the Industrial Trent gas turbine with an OTSG offers a fast start, highly flexible CCGT plantdue to the ability of the OTSG to run dry and its dynamic heating surface allowing fast flexible operation.The low exhaust gas temperatures of the Industrial Trent are well suited to an OTSG, while an addedbenefit of using an OTSG is the reduction in make-up water volumes required as no steam blow-down isrequired. This combination ensures no life impact on any components due to frequent starting andstopping, with the possibility to undertake several hundred starts per year. The compact nature of anOTSG also complements the modular package design of the Industrial Trent to create a compact, lowfootprint, fast build multi-unit power plant.

From cold, it is possible for a 2 on 1 Trent DLE CCGT to reach 100% plant load within 40 minutes of startinitiation, with 80% of station power available from the two gas turbines within 10 minutes of start-up.

Figure 11: Industrial Trent DLE ‘2 on 1’ CCGT Start Curve

A duct burner can be added to each OTSG to boost steam production and power output. As can be seenin Figure 11 above, once the steam turbine has reached full load, duct firing can be switched on and thetotal power output increased by up to 50% within 10 minutes to further increase the operational flexibilityof the power plant. Using a 2 on 1 configuration, this enables a power plant to operate efficiently and withlow emissions from around 20% of rated station load to 150% of rated station load, with the ability tochange loads rapidly and frequently during operation.

In all instances, the CCGT configurations discussed achieve net plant efficiencies in excess of 50%% witha high degree of flexibility in operation, maintenance and economic decisions.

Figure 12: Industrial Trent DLE ISI Embedded Flexible CCGT diagram

4.2 ‘Single Shaft’ 1+1 CCGT

For single unit sites, or for sites where expansion is foreseen to be phased over a number of years, asingle shaft option could be considered. By having a double end drive alternator with the steam turbinecoupled to the alternator via a clutch mechanism, it produces a very compact CCGT plant with a highpower density. This configuration also enables the gas turbine to operate in simple cycle for low peakingdemand, increasing the operational flexibility of the power plant. As for the conventional CCGTconfiguration, a duct burner can be installed if required to further boost power generation, enhancing theplant flexibility still further.

4.3 ‘Dry’ CCGT

In an increasing number of locations around the World, water is deemed a scarce resource and powergeneration must compete with industry, agriculture and people for this resource. Therefore there isincreasing pressure to move away from water cooling and steam cycles requiring make-up water andseek out alternative ‘dry’ combined cycle technologies, such as Organic Rankine Cycle (ORC) orSupercritical CO2 (SCO2) with Air Cooled Condensers .

While perhaps not offering yet the same level of efficiency at full load as conventional steam combinedcycle, significant efficiency improvements can be achieved compared to open cycle operation at lowercost than a high pressure (HP) steam system. The cost of the steam cycle can be reduced by using a lowpressure steam system (LP) with reduced steam temperatures to a similar cost level as ORC and SCO2,but the efficiency falls to a similar level as well.

Due to the flatter efficiency characteristics of the ORC and SCO2 turbines, the ‘dry’ solutions tend to havebetter part load efficiencies than these LP steam designs, an important feature for a flexible power plantwhich may be required to operate at part loads for long periods of time. While not an efficiency optimisedsolution, acceptably high efficiencies can be achieved even at high ambient temperatures, with the benefitof requiring zero water. The working fluids used in these ‘dry’ combined cycle solutions also eliminate thepotential for condensation within the turbine, and as closed systems require no make-up fluid or fluidtreatment, unlike water which requires considerable treatment to meet the quality requirements of theboiler and steam turbine. This enables the ‘dry’ solutions to offer lower operating and maintenance costscompared to steam, another benefit to offset the efficiency reduction.

Figure 13: Comparison of efficiency for an Industrial Trent in open cycle and with an ORC system at a40°C ambient temperature

An interesting feature of ORC is that the turbines rotate at relatively low speeds. For an ORC turbinematched to the exhaust gas conditions of the Industrial Trent, the speed of rotation is typically 3000rpm,the same speed as the LP turbine and alternator. This opens the possibility to develop a single shaftconcept as for the conventional steam combined cycle concept discussed earlier, offering the potential toreduce the CAPEX of this configuration and increase the efficiency compared to a ‘standalone’ solution.

An ORC system can also be kept in ‘hot standby’ condition by installing a small gas burner to maintainthe working fluid at an elevated temperature. By also utilising the exhaust gases of the gas turbine duringthe start-up phase, the ORC system can be brought on line more quickly, and full load achieved within 10minutes of gas turbine start. Thus by combining the Industrial Trent with an ORC turbo-generator, fullplant load can potentially be achieved within 10 minutes of start-up, and similarly fast shutdowns can beachieved. Using this concept, the ORC solution may actually be the most economic option under the‘pulse load’ scenario described in section 3.1 even though it is not as efficient as the conventionalcombined cycle configurations.

Conclusions

With generators and system operators facing ever changing challenges as renewable power generationincreases, it is imperative that their assets are as flexible as possible in order to maximise their economicviability while still ensuring supply security and correct system operation.

A power plant based on an aero-derivative gas turbine such as the Industrial Trent offers not onlyoperational flexibility due to the gas turbine’s specific characteristics, such as its ability to operate withoutpenalty in high stress cyclic operating cycles, but also in the range of power plant configurations that canbe adopted, allowing a single power plant to operate economically as a either peaking plant, a mid-meritplant or a base load plant. The gas turbine features and plant concepts also allow power to bedespatched quickly when required, with both open cycle and combined cycle configurations able to putpower on the bars in between the 10 and 30 minute gate closure times required by markets and systemoperators.

Acknowledgements

The author would like to thank his colleagues John Charlton, Andy Buckenberger, Jonathan Li andThorsten Krol for their support in writing this paper.

References

1. The Case for Flexible Embedded Power Generation, John Charlton, Siemens, IDGTEConference, Milton Keynes, UK, November 2015

2. Maximising Profits through efficient Pulse Load Operation, Christian Hultholm and Jame Lopez,Wartsila, Power-Gen Natural Gas, Columbus, OH, August 2015

3. CHP: Maximizing Energy Efficiency, Minimizing Environmental Impact and Reducing OperationalCosts, Michael Welch, Siemens, Electric Power 2016, New Orleans, LA, April 2016

4. Improving the Flexibility and Efficiency of Gas Turbine-based Distributed Power Plant, MichaelWelch & Andrew Pym, Siemens, Power-Gen Natural Gas, Columbus, OH, August 2015

5. Flexible Combined Cycle Gas Turbine Power PLANT UTILISNG Organic Rankine CycleTechnology, Michael Welch, Siemens, and Nicola Rosetti, Turboden, ASME Turbo EXPO 2016,Seoul, South Korea, June 2016

Nomenclature

CAPEX Capital Expenditure

CC Combined Cycle

CCGT Combined Cycle Gas Turbine

DLE Dry Low Emissions

HP High Pressure

HRSG Heat Recovery Steam Generator

ICE Internal Combustion Engine (Gas Engine)

ISI Inlet Spray Intercooling

LP Low Pressure

O&M Operation and Maintenance

OC Open Cycle

OEM Original Equipment Manufacturer

ORC Organic Rankine Cycle

OTSG Once Through Steam Generator

SCO2 Supercritical Carbon Dioxide

WLE Wet Low Emissions (water injection)